Sea-level rise of 20 to 80 feet will be all but unstoppable by midcentury if current emissions trends continue. The first few feet of sea-level rise alone will displace more than 100 million people worldwide and turn all of our major Gulf and Atlantic coast cities into pre-Katrina New Orleans–below sea level and facing superhurricanes.

How fast can seas rise? For the past decade, sea levels have been rising about 1 inch a decade, double the rate of a few decades ago. The Third Assessment Report of the U.N. Intergovernmental Panel on Climate Change (IPCC), released back in 2001, projected that sea levels would rise 12 to 36 inches by 2100, with little of that rise coming from either Greenland or Antarctica. Seas rise mainly because ocean water expands as it gets warmer, and inland glaciers melt, releasing their water to the oceans.

Sea-level rise is a lagging indicator of climate change, in part because global warming also increases atmospheric moisture. More atmospheric moisture probably means more snowfall over both the Greenland and Antarctica ice sheets, which would cause them to gain mass in their centers even as they lose mass at the edges. Until recently, most scientists thought that the primary mechanism by which these enormous ice sheets would lose mass was through simple melting. The planet warms and ice melts–a straightforward physics calculation and a very slow process, with Greenland taking perhaps a thousand years or more to melt this way, according to some models.

Since 2001, however, a great many studies using direct observation and satellite monitoring have revealed that both of the two great ice sheets are losing mass at the edges much faster than the models had predicted. We now know a number of physical processes can cause the major ice sheets to disintegrate faster than by simple melting alone. The whole idea of “glacial change” as a metaphor for change too slow to see will vanish in a world where glaciers are shrinking so fast that you can actually watch them retreat.

The disintegration of the Greenland and Antarctic ice sheets is a multistage process that starts with the accelerated warming of the Arctic….